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$ \begin{aligned}解:原式&=b•\frac ba \\ &=\frac{b^2}{a} \\ \end{aligned}$

$ \begin{aligned}解:原式&=(x-2)÷\frac {2-x}{x}•x \\ &=(x-2)×(-\frac {x}{x-2})•x \\ &=-x^2 \\ \end{aligned}$

$ \begin{aligned}解:原式&=\frac {1}{m+1}×\frac {(m+1)(m-1)}{(m-1)^{2}}-\frac {2}{m-1} \\ &=\frac{1}{1-m} \\ \end{aligned}$

$ \begin{aligned} 解:原式&=\frac {x}{(x+1)(x-1)}÷\frac {x}{x+1} \\ &=\frac {x}{(x+1)(x-1)}×\frac {x+1}{x} \\ &=\frac{1}{x-1} \\ \end{aligned}$

$ \begin{aligned} 解:原式&=\frac {3a(a+3)-a(a-3)}{a^2-9}×\frac {a^2-9}{a} \\ &=\frac {3a^2+9a-a^2+3a}{a^2-9}×\frac {a^2-9}{a} \\ &=2a+12 \\ \end{aligned}$

$ \begin{aligned} 解:原式&=\frac {m}{(m+2)(m-2)}÷\frac {m}{m-2} \\ &=\frac {m}{(m+2)(m-2)}×\frac {m-2}{m} \\ &=\frac{1}{m+2} \\ \end{aligned}$

$ \begin{aligned} 解:原式&=\left[-\frac {3}{x-2}-\frac {(x+2)(x-2)}{x-2}\right]×\frac {(x-2)^2}{x-1} \\ &=-\frac {3(x-2)}{x-1}-\frac {(x+2)(x-2)^2}{x-1} \\ &=-\left[\frac {(x-2)(3+x^2-4)}{x-1}\right] \\ &=-\left[\frac {(x-2)(x+1)(x-1)}{x-1}\right] \\ &=-x^2+x+2 \\ \end{aligned}$

$ \begin{aligned} 解:原式&=\frac {3ab}{8c}•(-\frac {2a}{b}) \\ &=-\frac{3a^2}{4c} \\ \end{aligned}$